Scientists see the light on the 'weirdest' fossil

Scientists will unravel the riddle of the most surreal animal from the weirdest phase of the Earth's history today.

When the first fossils of Hallucigenia were discovered a century ago in the famous Burgess Shale deposits of Canada, its appearance astonished scientists who could find no modern equivalent in the animal kingdom.

The fossilised imprints of the creature, which lived on the floor of an ancient lake some 500 million years ago, suggested that it stood on several pairs of stilt-like legs and grew a set of waving tentacles on its back.

Even when researchers named and formally described Hallucigenia in 1977 they were so confused about its appearance that they mistakenly sketched it upside down and back to front – the "tentacles" were in fact its legs and the "legs" were really spines on the creature's back.

In 1995, further research suggested that Hallucigenia's globular "head" was in fact its rear end and its real head was at the narrow end. Now one of the world's specialists in Burgess Shale fossils has found that there is two forms of this Dali-esque creature.

Desmond Collins, a palaeontologist at the Royal Ontario Museum in Toronto, will tell the annual meeting of the Palaeontological Association at Cambridge University that Hallucigenia may have had male and female forms.

Dr Collins said that fresh specimens of Hallucigenia suggest that that there is a larger and more robust form of Hallucigenia with "a rigid trunk, robust neck and a globular head". The smaller form is thinner, with a flexible trunk, a small head with two fang-like projections, two short horns and possibly a pair of eyes connected to the trunk by a very thin neck, he said.

"Both forms have seven pairs of robust spines along the back, and seven pairs of long, thin, flexible legs terminating in the large claw typical of onychophorans [modern caterpillar-like invertebrates]," Dr Collins said.

Hallucigenia became famous following the publication of Stephen Jay Gould's award-winning book of 1989, Wonderful Life, which vividly described the variety of weird creatures living during the Cambrian period. Most fossils are of animals with shells or bones but the Burgess Shale is one of the few sites where soft body parts were well preserved. Other fossils include Wiwaxia, a scaly creature with a circular array of spines on its back, and Anomalocaris, a fierce squid-like predator.

Although Gould's widely read account of the Burgess Shale brought the arcane subject to a mass audience, he managed to repeat the original mistake over Hallucigenia's upside-down, back-to-front appearance. Gould claimed that Hallucigenia's oddness showed that Cambrian-era fossils are unrelated to anything alive today. He believed that most became extinct without leaving living descendants. But palaeontologists now believe that Hallucigenia was the ancestor of some of the most successful groups of modern-day animals – the arthropods – which include insects, spiders and crabs.

Instead of being odd off-shoots with no relevance to living animals, Hallucigenia and its contemporaries have features that can be linked with some of the most successful living organisms, said Graham Budd, an assistant professor of palaeontology at the University of Uppsala in Sweden.

"Rather than being weird oddities that tell us nothing about modern animals, I think these animals, by possessing some, but not all of the features of the modern animals, actually tell us the routes by which modern animals were assembled," Professor Budd said.

"The new material shows that the 'classical' Hallucigenia is not a weird one-off, but representative of a rich diversity of Cambrian animals. The world has moved on enormously since the publication of Wonderful Life when these animals were seen as weird offshoots of a huge explosion in biological diversity, most of which went extinct."

It is now possible to see Hallucigenia as part of a series of animals that progressively show more and more features of one of the major groups of today – the arthropods," he said.